Communications switching and signalling networks are subject to congestion and overload when the offered traffic is above the capacity of the network to handle the load and various techniques have been developed to control such overload and congestion for particular situations. One such control system which utilizes a call-gapping algorithm to control traffic volume in the system is disclosed in U.S. Pat. No. 5,060,258 entitled "Call Traffic Control" by Peter M. D. Turner, to which the reader is directed for reference. For further details on the application of this algorithm, as well as an excellent review of two prior algorithms, the reader is directed to a paper entitled "A New Call Gapping Algorithm for Network Traffic Management" by P. M. D. Turner and P. B. Key, 13th International Teletraffic Congress, Copenhagen (1991) volume 14, pp. 121-126. The contents of both of these documents are incorporated herein by reference.
In these existing call-gapping algorithms, the volume of calls allowed through the system, is always at or below the volume limit with very high offered traffic. However, it is desirable in some cases within operating communications networks to be able to firmly limit call volumes under conditions of heavy overload, while still allowing some greater volume when the degree of system overload is smaller. Such cases can arise, for example, when the network of a different service provider is interconnected to allow an exchange of calls. The expected traffic volumes may not be well estimated and it is desirable to carry as much of the traffic as reasonable. Under heavy overload, however, it is desirable to firmly limit the accepted traffic to achieve better fairness in the completion of calls from different sources. With the methods of the prior art, the operators of the communications system would have to change the control parameters as the volume of offered traffic varies in order to achieve this end.